1. Field of the Invention
The present invention relates to a process for carrying out a highly exothermic reaction such as that between an olefin and an organic hydroperoxide using a solid catalyst to form an oxirane compound.
2. Description of the Prior Art
Substantial difficulties are encountered in carrying out highly exothermic reactions where reactants and/or products are temperature sensitive. For example, the liquid phase reaction of propylene and an organic hydroperoxide using a solid catalyst to produce propylene oxide is a highly exothermic reaction, and selectivity to the desired product is very temperature sensitive. Proper control of reaction temperature presents a serious problem.
Conventional reactors for exothermic reactions are usually of two types:
(1) Quench type which consist of multiple fixed beds with cold feed quench injected in between beds
(2) Tubular type in which the catalyst is placed in the tubes of vertical shell and tube heat exchanger.
If the heat of reaction is high, the first type does not provide sufficient heat removal and proper reaction temperature control may not be possible.
The tubular reactor cost becomes prohibitive when high heats of reaction have to be removed through heat exchanger surfaces operating with a low heat transfer coefficient. There is also a temperature gradient from the center of the tube which is often detrimental to a process which requires nearly isothermal conditions.
Epoxidation can be carried out using multiple fixed catalyst bed reactors. The fixed bed epoxidation process may be practiced with a fresh bed last or fresh bed first rotation plan. See U.S. Pat. No. 5,849,937. Fresh bed first is preferable, since the downstream, older beds can be run at a higher temperature. This obtains the maximum activity at the best selectivity and with a minimum heat input and capital expense. One problem with fresh bed first however, is that of temperature control. The temperature rise for an adiabatic bed is large, about 150xc2x0 F. or more and this results in rapid catalyst deactivation of the downstream portion of the bed. It also means that the front part of the bed, where the temperature is much cooler, is not converting very much product. A second problem is that the fixed bed reactors are very difficult to operate with normal reactant concentrations. If one wishes to obtain 30% to 50% hydroperoxide conversion, this is essentially impossible in an adiabatic bed because the reactor exhibits multiple steady states. One can obtain 1 to 15% hydroperoxide conversion or 99.0 to 99.9% conversion, but obtaining 30 to 50% hydroperoxide conversion is not possible.
To illustrate this, reference is made to FIG. 1 which is a plot of reaction inlet temperature versus hydroperoxide conversion for a conventional reaction system for propylene oxide production by reaction of propylene and ethylbenzene hydroperoxide. As can be seen, there is a steady increase in hydroperoxide conversion with increasing inlet temperature until an inlet temperature is reached at which hydroperoxide conversion jumps from a relatively low level to nearly 100%. When inlet temperature is then reduced, hydroperoxide conversion remains near 100% until at a substantially lower temperature hydroperoxide conversion suddenly falls to a much lower level. Under normal conditions, control of conversion at an intermediate level e.g. 50%, is almost impossible to accomplish. This can be seen from FIG. 1. When the inlet temperature is raised, the conversion suddenly jumps from 20 to 99%. Upon reducing the inlet temperature, the conversion suddenly drops from 99 to 2%.
In accordance with the present invention, the exothermic reaction between an olefin such as propylene and an organic hydroperoxide such as ethylbenzene hydroperoxide using a solid catalyst is carried out while maintaining the concentration of hydroperoxide in the feed below 8 wt % by either diluting the feed with a process stream depleted in hydroperoxide or by using a plurality of epoxidation zones and feeding only a portion of the total hydroperoxide feed to each zone.
In accordance with one embodiment, the invention a reactor system is provided comprised of a series of reaction zones packed with solid catalyst. The reaction mixture from the first reaction zone is separated into two portions, one portion being recycled to the feed to the first zone, the remainder passing to the second zone. The recycled portion is admixed with cold feed thus both preheating and diluting the feed while moderating the temperature of the reaction mixture passing through the first reaction zone and permitting convenient control of reactant conversion at a desired intermediate level.
In accordance with another embodiment, again a series of reaction zones is used with a portion of the total hydroperoxide being fed to each zone.